The term liquid is here taken to include liquids having non-Newtonian properties. The viscous liquid will hereinafter be referred to as oil, the liquid with a lower viscosity as water.
A known application of the above-mentioned process is the transport of oil through pipelines, water or a liquid phase substantially consisting of water being used as the liquid with a lower viscosity. Here the oil moves in the form of a long plug through the center of the pipeline, the water forming a layer between the oil and the wall of the pipeline. Since the friction between water and wall is much lower than that between oil and wall, the resistance to flow is considerably smaller than in the pumping of oil only. In this way it is even possible for very stiff oils or oils of temperatures lying below the pour point to be transported through a pipeline.
In this connection it is of great importance that no oil drops are present in the water besides the central oil plug. Oil drops dispersed in the water layer would considerably increase the thickness of that layer neccessary to separate the central oil plug from the wall. As a result a smaller part of the volume transported would consist of oil, while in addition the separation between water and oil after transport would be more difficult.
It has been found that the formation of oil drops occurs there where water and oil are added one to the other; oil drops then form more readily when the viscosity of the oil is lower.
As a result of the presence of the annular outlet for the water round about the exit for oil, immediately after oil and water having been added one to the other an annular layer of water is formed around the oil. The formation of oil drops is in the first place determined by the local average velocity of flow of the oil. The higher that velocity, the greater the chance of drops being formed.
In the present process of transport the average velocity of flow in the pipeline is as a rule higher than 0.5 m/s. This lower limit is related to the phenomenon that at lower flow velocities there is a risk of the oil plug penetrating through the annular layer of water and contacting the wall of the pipline, which is undesirable.
Oil flow velocities higher than 0.5 m/s, however, often give rise to the formation of drops when the annular layer of water is formed. Peripheral effects in the velocity profile of the oil play an important role here.